Acoustical panel comprising interlocking matrix of set gypsum and method for making same

ABSTRACT

An acoustical panel comprising a continuous phase of an interlocking set gypsum matrix and a method of preparing an acoustical panel are disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.60/455,782, filed Mar. 19, 2003, which is hereby incorporated byreference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to acoustical panels. Moreparticularly, the invention relates to acoustical panels comprising aninterlocking matrix of set gypsum and to methods for the preparationthereof.

BACKGROUND OF THE INVENTION

Acoustical panels are used to form interior surfaces, such as ceilingtiles, wall panels, and other partitions (e.g., partitions betweenoffice cubicles), in commercial or residential buildings. The panels aregenerally planar in shape and include an acoustical layer containing acombination of materials selected to provide suitable acousticabsorbency while retaining sufficient durability. For example, commonmaterials presently used in forming acoustical panels include mineralwool, fiberglass, expanded perlite, clay, calcium sulfate hemihydrate,calcium sulfate dihydrate particles, calcium carbonate, paper fiber, andbinder such as starch or latex. Mineral wool is most commonly usedbecause it helps create a porous fibrous structure and thus providesgood sound absorption.

Many acoustical panels are prepared in a manner similar to conventionalpapermaking processes by water-felting dilute aqueous dispersions ofmineral wool, perlite, binder, and other ingredients as desired. In suchprocesses, the dispersion flows onto a moving foraminous support wire,such as that of a Fourdrinier or Oliver mat-forming machine fordewatering, as will be appreciated by one of ordinary skill in the art.The dispersion dewaters first by gravity drainage and then by vacuumsuction. The wet mat is dried in a heated convection oven, and the driedmaterial is cut to desired dimensions and optionally top-coated withpaint to obtain the finished panel. An example of a panel prepared inthis manner is the AURATONE® ceiling tile, commercially available fromUSG Interiors, Inc.

Acoustical panels also can be made by a wet pulp molding or cast processsuch as described in U.S. Pat. No. 1,769,519. In accordance with thisprocess, a molding composition comprising granulated mineral woolfibers, fillers, colorants, a binder such as cooked starch, and water,is prepared for molding or casting the panel. The composition is placedupon suitable trays that have been covered with paper or a paper-backedmetallic foil and then the composition is screeded to a desiredthickness with a forming plate. A decorative surface, such as elongatedfissures, also may be provided by a screed bar or patterned roll. Thetrays filled with the mineral wool composition are then placed in anoven to dry. An example of a panel prepared in this manner is theACOUSTONE® ceiling tile, commercially available from USG Interiors, Inc.

The water felting and tray casting techniques for preparing acousticalpanels are not entirely satisfactory because of their complexity andrather significant expense. In addition to raw material costs, theseprocesses expend large amounts of water and energy. Furthermore, manypanels prepared according to these methods are subject to unsightlypermanent deformation such as sag, especially under conditions of highhumidity. In this respect, because many panels are composed ofhygroscopic binder such as recycled paper fiber and/or starch, suchpanels are susceptible to sagging. The possibility of sagging isparticularly problematic in the event that the panels are stored and/oremployed horizontally. The panels may sag, for example, in areas betweenthe points at which they are fastened to, or supported by, an underlyingstructure, including, for example, a ceiling grid. The problem ofsagging can be more pronounced where the panels must carry loads,including, for example, insulation.

Some acoustical panels are designed to have set gypsum (i.e., calciumsulfate dihydrate) in the acoustical layer. Because set gypsum is notinherently a particularly acoustically absorbent material, manyacoustical panels comprising set gypsum include very largemechanically-formed holes that may be, for example, drilled, punched, orotherwise formed to pass through the entire depth of the panel. Theholes of acoustical panels of this type typically have a diameter of atleast one centimeter, such as found in acoustical panels commerciallyavailable from Danoline of Valby, Denmark and from British Gypsum. Manyof these panels also utilize an acoustically functional backing sheet.The acoustically functional backing sheet is typically glass fleece or apolymeric material that absorbs or dissipates sound transmitted by thelarge mechanically-formed holes, but a significant amount of sound isstill transmitted through the panel. Although the largemechanically-formed holes provide some acoustical absorbance where thereis a plenum behind the panel, many consumers do not find them to beaesthetically pleasing. Gypsum-based acoustical panels havingmechanically formed large holes also are relatively dense products andtherefore are cumbersome to transport, to handle, and to install. Inaddition, the backing sheet adds considerable expense to such products.

More recently, there have been efforts in the art to form acousticalpanels from cementitious materials. For example, U.S. Pat. No. 6,443,258B1 describes an acoustically absorbent porous panel formed individuallyin a mold from a cured aqueous foamed cementitious material thatincludes a very low ratio of water to cementitious material (i.e., lessthan 1:1). Paper fibers are avoided in the panels described in the '258patent so as to accommodate the low water to cementitious material ratiotaught therein. As a substitute for paper fiber, the '258 patentdescribes the use of polyester, fiberglass, or mineral woolcharacterized by very long fiber lengths. According to the '258 patent,the length of such fibers is preferably on the order of 0.5 inches sothat the fibers can pass through the crystalline cement structure andalso pass through the pores created by the foaming process. Thus,acoustical panels prepared according to the '258 patent are expensive toproduce and the process for making the panels is inefficient forgenerating economies of scale.

Accordingly, it will be appreciated from the foregoing that there is aneed in the art for acoustical panels that are relatively inexpensive tomanufacture and that are produced efficiently in large quantities on agypsum board line. It will be appreciated also that there is a need inthe art for such an acoustical panel that is aesthetically pleasing anddoes not require the presence of relatively large holes that aremechanically bored into the panel. It will be appreciated further thatthere is a need in the art for such an acoustical panel that resistspermanent deformation, such as sag. The invention provides an acousticalpanel and method for the preparation thereof that includes suchfeatures. These and other advantages of the present invention, as wellas additional inventive features, will be apparent from the descriptionof the invention provided herein.

BRIEF SUMMARY OF THE INVENTION

The present invention provides acoustical panel comprising an acousticallayer comprising an interlocking matrix of set gypsum. In someembodiments, the panel is the acoustical layer by itself as a monolithicproduct. In other embodiments, the panel comprises a composite whichincludes the acoustical layer, a backing sheet, and optionally a supportor densified layer disposed between the backing sheet and acousticallayer. In other embodiments, the panel includes a scrim layer disposedbetween the densified layer and the acoustical layer. Variousalternative combinations of acoustical layer, support or densifiedlayer, scrim layer, and backing sheet are also contemplated.

The present invention also provides a method for preparing acousticalpanel. In one embodiment of the method of the present invention, amixture comprising water, foaming agent, and calcined gypsum is cast toform an acoustical layer precursor as part of a continuous ribbon ofpreselected width and thickness. The cast acoustical layer precursor ofthe ribbon is maintained under conditions sufficient for the calcinedgypsum to form an interlocking matrix of set gypsum, and thereby formsthe acoustical layer. Typically the acoustical layer is wet at thisstage of the process due to the presence of excess water in the castribbon from which the interlocking matrix of set gypsum is formed. Thewet acoustical layer is cut to form wet acoustical panel precursor ofpre-selected dimensions. Wet panel precursor is dried to form a dryacoustical panel. In some embodiments, the acoustical panels of thepresent invention exhibit a Normal Incident Sound Absorption of at leastabout 0.32, pursuant to modified ASTM E 1050-98, as described hereinbelow.

Preferably the continuous ribbon includes a backing sheet for supportingthe acoustical layer precursor during the preparation of acousticalpanel. Typically, the backing sheet remains a part of the finishedacoustical panel, but it need not in all embodiments of the invention.In preparing acoustical panel, the mixture for forming the acousticallayer precursor is applied directly to the backing sheet. In preferredembodiments, a mixture for forming the densified layer precursor isapplied to the backing sheet prior to application of the acousticallayer precursor. In other embodiments, a scrim layer is applied betweenthe densified layer and the acoustical layer precursor.

In some embodiments, the present invention provides a method forpreparing acoustical panel comprising preparing a continuous ribboncomprising a mixture for forming the acoustical layer, the mixtureincluding (a) water, (b) calcined gypsum, and (c) foaming agent, andoptionally one or more of the following: (d) cellulosic fiber, (e)lightweight aggregate, (f) binder, (g) accelerator, (h) water reducingagent, and (i) an enhancing material selected from the group consistingof an ammonium polyphosphate having 500-3000 repeating phosphate units,a trimetaphosphate compound, a tetrametaphosphate compound, ahexametaphosphate compound, and combinations thereof. Backing sheet,densified layer, and/or scrim layer can be included as described herein.The ribbon is maintained under conditions sufficient for the calcinedgypsum to form an interlocking matrix of set gypsum. The ribbon is cutto form one or more panels of pre-selected dimensions.

In some embodiments, the present invention provides acoustical panelcomprising an acoustical layer comprising an interlocking matrix of setgypsum and one or more additives such as cellulosic fiber, lightweightaggregate, and/or an enhancing material selected from the groupconsisting of an ammonium polyphosphate having 500-3000 repeatingphosphate units, a trimetaphosphate compound, a tetrametaphosphatecompound, a hexametaphosphate compound, and combinations thereof.Binder, foaming agent, accelerator, and water reducing agent can also beincluded in the mixture used to form the acoustical layer of theacoustical panel. Preferably, the panel comprises a backing sheet forsupporting the acoustical layer. Even more preferably, the panelcomprises a densified layer on the backing sheet, and still morepreferably, the panel includes a scrim layer between the densified layerand the acoustical layer.

The invention may best be understood with reference to the followingdetailed description of the preferred embodiments.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides acoustical panel which includes anacoustical layer comprising an interlocking matrix of set gypsum, and acontinuous process for making acoustical panel. The acoustical panel canbe the acoustical layer itself as a monolithic layer and it can be amulti-layer composite. Panels according to the invention exhibitdesirable acoustical properties, flexural strength, surface hardness,and resistance to permanent deformation, such as sag, for use in avariety of different applications, including, for example, ceilingtiles, wall panels, and partitions such as between office cubicles orthe like. In keeping with these and other applications, the acousticallayer of the inventive acoustical panels preferably has a density offrom about 10 lb/ft³ to about 25 lb/ft³, more preferably, from about 12lb/ft³ to about 20 lb/ft³, with a density of about 16 lb/ft³ mostpreferred in ceiling tile applications. In some embodiments without adensified layer and/or a scrim layer as described herein, it may bedesirable to provide an acoustical layer having a density at the higherend of the preferred range, or even higher.

Advantageously, the set gypsum-containing acoustical panels of theinvention provide the desirable acoustical properties without requiringthe presence of large, mechanically generated holes, and without the useof mineral wool. Thus, in preferred embodiments, the panels areessentially free of mineral wool and mechanically generated holes havinga diameter greater than 1 cm such that the acoustical properties of theinvention are achieved without the presence of either, although suchmechanically generated pores and/or mineral wool can be included in someembodiments, if desired. Preferred panels according to the invention donot include voids of any kind (such as mechanically generated holes)having an average maximum diameter greater than about 2 mm.

In accordance with the present invention, acoustical panel can beprepared on a gypsum wallboard manufacturing line. A conventional gypsumwallboard manufacturing line can be used to make acoustical panel of thepresent invention, modified as desired to accommodate the strength andcure characteristics of the slurry used to make acoustical panel. Inmanufacture, ribbon comprising an acoustical panel precursor isprepared. In some embodiments, ribbon also includes a backing sheet,densified layer precursor and/or scrim layer. The acoustical layerprecursor and densified layer precursor (if present) used to form setgypsum-containing acoustical panel according to the invention areprepared by forming a mixture of water, calcined gypsum and otheradditives. Various components, such as, for example, cellulosic fiber,lightweight aggregate, binder, foaming agent, foam stabilizer,accelerator, water reducing agent, and/or enhancing material forimproving resistance to permanent deformation, e.g., due to highhumidity conditions, can be added to the aqueous calcined gypsum slurryas desired. The mixture for forming the densified layer precursor can bethe same or different in composition as the mixture for forming theacoustical layer precursor.

In the continuous process for making acoustical panel, the mixture usedfor forming acoustical layer is cast as an acoustical layer precursor inthe form of at least part of a continuous ribbon. The conveyor carriesthe ribbon on a conveyor, such as a belt, and/or rollers, or the like toa knife where the ribbon is cut into wet panel precursor of preselecteddimensions. The acoustical layer precursor and densified layer precursor(if present) are cured or hardened so as to form an interlocking matrixof set gypsum and hence are transformed into the acoustical layer anddensified layer (if present) as the ribbon travels to the knife. Thepresent invention contemplates the use of conveyors, rollers, andcombinations thereof, so long as the integrity of the cast ribbon ismaintained during the hardening or curing process leading to theformation of the interlocking matrix of set gypsum. In preferredembodiments, as acoustical layer forms, no outer high density boundarylayer forms contrary to the situation described with respect to thepanels of U.S. Pat. No. 6,443,258 B1. Such preferred embodiments of theinvention are advantageous because the panel is prepared according tothe process with no need to remove, such as by grinding, anynon-acoustical high density boundary layer.

The backing sheet can be provided to support the panel as is known inwallboard manufacture. The backing sheet desirably acts to transferstresses so that they are averaged out across the length of the ribbonor panel. The mixture for forming acoustical layer precursor can beapplied directly or indirectly onto the backing sheet. In preferredembodiments, the mixture for forming the densified layer precursor isapplied to the backing sheet to enhance strength. In even more preferredembodiments, to further enhance wet and dry strength of the acousticalpanel precursor and of the acoustical panel, a scrim layer is included.The scrim layer is applied, during formation of the panel, onto thesurface of the densified layer precursor opposite the backing sheet. Theacoustical layer precursor then is applied to the scrim layer.

As will be appreciated by one of ordinary skill in the art, the aqueousgypsum mixture (e.g., slurry) for forming the acoustical layer precursorand densified layer precursor (if present), respectively, can be formedin a conventional mixing apparatus used in making gypsum wallboard.After mixing, the aqueous gypsum mixture used to form the densifiedlayer precursor, if present, is discharged from the mixer from one ormore outlets separate from the outlet for the aqueous gypsum mixtureused to form the acoustical layer precursor. The substrate onto whichthe gypsum mixture is cast will depend on the type of panel being made.For example, the mixture for forming the acoustical layer precursor canbe cast onto (a) the backing sheet; (b) the scrim layer, if present; or(c) the densified layer precursor, if present and the scrim layer is notpresent.

The backing sheet is rolled onto a moving surface (such as a conveyor orthe like) to cast a continuous ribbon of desired shape comprising theacoustical layer precursor, the backing sheet, and optionally, one orboth of the densified layer precursor and scrim layer. If desired, aforming plate or roller can be applied to the outer surface of theacoustical layer precursor to achieve a desired thickness. It also willbe appreciated that a facing sheet can be applied to the acousticallayer precursor. When a facing sheet is employed, the forming plate orroller is applied to the facing sheet to arrive at the selectedthickness.

The acoustical layer is formed after the acoustical layer precursorcures, i.e., after the interlocking matrix of set gypsum is formed.Desirably, the acoustical layer has a thickness of from about 0.3 inchesto about 0.75 inch, preferably about 0.375 inch, about 0.5 inch, about0.625 inch, about 0.750 inch, about 1 inch, or thicker). Where thedensified layer and/or scrim layer are included in the acoustical panel,lower thickness for the acoustical layer can be selected from thesepreferred ranges, as will be appreciated by one of ordinary skill in theart. Acoustical panel of the invention preferably has an overallthickness of from about 0.5 inch to about 1 inch, more preferably fromabout 0.5 to about 0.625 inch.

The use of a forming plate or forming rollers is well known in theproduction of gypsum wallboard. In some embodiments of the presentinvention, for example, where no facing sheet is present, it may bedesirable for the forming plate or rollers to be in the form of avibrating plate or a fluidization membrane. The vibrating plate isvibrated horizontally so as to create shear, thereby preventing thegypsum slurry from adhering to the vibrating plate, and thus allowingfor a smooth uniform caliper.

The fluidization membrane permits water to be injected through pores inthe membrane at a very low flow rate, thereby forming a slick surfacebetween the forming fluidization membrane and the gypsum slurry, so asto inhibit the adherence of gypsum slurry onto the forming fluidizationmembrane and allowing for a smooth uniform caliper. By way of example,the fluidization membrane can be in the form of Dynapore Models LFM-1 orLFM-10, commercially available from Martin Kurz & Co., Inc. of Mineola,N.Y.

In preferred embodiments, a fluidization device comprises a fluidizationmembrane, which is disposed so that a face surface of the fluidizationdevice contacts the gypsum slurry to control ribbon thickness. Thefluidization device also includes at least one sidewall (e.g., foursidewalls) extending upwardly and away from the gypsum slurry. Means forproviding water from a water source, e.g., a hose, tube, or the like,onto a back surface of the membrane is also provided. The water thenseeps through the pores from the back surface of the fluidizationdevice, to the face surface of the fluidization device, and onto thegypsum slurry. In some embodiments, the fluidization device alsoincludes a top wall connecting the sidewalls and disposed parallel tothe fluidization membrane so as to form a box, so long as an opening iscreated in the top wall or sidewalls to allow for the means forproviding water.

If desired, a pattern can be permanently impressed onto the continuousribbon, using conventional techniques, to provide a desired texture inacoustical panel for aesthetic purposes. For example, a continuous beltmeans can be used to impress a pattern as the gypsum sets, as is knownin the art. The pattern can be impressed in the acoustical layerprecursor. As will be appreciated by one of ordinary skill in the art,the pattern preferably is impressed at a point in the setting process atwhich sufficient setting has taken place so that the pattern isretained.

The continuous ribbon continues to travel from the rollers to a knife,where the ribbon is cut into wet panel precursors of a predeterminedlength. If desired, two or more knives, applied at different stages inthe process, can be used to cut the ribbon into precursors ofpredetermined dimensions suitable for drying (e.g., 4 ft×9 ft), as willbe appreciated by one of ordinary skill in the art. The knife can be inany suitable form, such as, for example, a conventional knife used inwallboard manufacture, a water jet knife, or the like. As the continuousribbon travels toward the knife, it is allowed to harden to form aninterlocking matrix of set gypsum, that is, a matrix of calcium sulfatedihydrate. It is the desired hydration of the calcined gypsum thatenables the formation of an interlocking matrix of set gypsum crystals,thereby imparting strength to the gypsum structure in acoustical panel,as well as green strength (discussed herein below) to the continuousribbon as it hardens and in the wet panel precursor.

The length of the continuous ribbon can vary. Because the aqueous gypsummixture typically is allowed to harden as the continuous ribbon istransported on the surface on which it is cast (e.g., a conveyor or thelike), the continuous ribbon may extend for 50 ft or longer, 100 ft orlonger, 250 ft or longer, and preferably, 350 ft or longer. For example,depending upon the design of the manufacturing line, based upon thespeed of the manufacturing line and rate of hydration of the gypsumslurry, the continuous ribbon can be as long as 500 ft or longer, 750 ftor longer, 1000 ft or longer, and 1250 ft or more in preferredembodiments so that at least about 98% of the set gypsum is formed priorto cutting the ribbon into wet panel precursor.

After the gypsum sets and the ribbon is cut, the wet panel precursorsare transferred to a kiln to dry off unreacted water under mild heat toyield dry acoustical panel. The wet panel precursors are dried for aperiod of time sufficient to dry the panels, preferably at a lowertemperature than temperatures used in drying conventional gypsumwallboard. In this respect, the panels preferably are dried at atemperature so as to avoid re-calcining the set gypsum, especially inembodiments where a facing sheet, which might otherwise protect thegypsum from re-calcining, is lacking. Because the acoustical layer ofpreferred panels according to the invention preferably has a relativelylow density, e.g., from about 10 lb/ft³ to about 25 lb/ft³, preferredpanels preferably require a drying time in the kiln of about 90 minutesor less.

In gypsum board lines where driven rollers are employed to transport thecut panel precursors to the kiln at higher rates than the movablesurface on which the gypsum is cast, it may be necessary to control themotors driving the transporting rollers to reduce the rate of transporttypically used with conventional gypsum wallboard in order to reducevibration to which the cut panel precursors might otherwise besubjected. It may also be desirable in some embodiments to providesufficient numbers of the transporting rollers so that they are spacedno more than about 6 inches apart, more preferably, no more than about 4inches apart (or even closer), and/or to use belts instead or on top ofthe transporting rollers, so that there is less opportunity for thepanel precursors (especially the ends) to become damaged by, or caughtin, the transporting rollers, and to dampen vibrations. In someembodiments, the entire line can be belted.

In preferred embodiments, panels according to the invention comprise abacking sheet. Any suitable material for the backing sheet may be usedsuch that the backing sheet provides support and strength for theribbon, panel precursors, and the acoustical panel. For example, thebacking sheet helps enhance resistance to vibration, thereby inhibitingfracture, as the ribbon and panel precursors are subjected to themanufacturing process. In some embodiments, the backing sheet may be inthe form of a material such as paper (e.g., manila paper, kraft paper,etc.), non-woven glass face, metallic foil (e.g., aluminum), and thelike. In some embodiments, a laminate containing paper and, for example,a metallic foil can be used, in which case the paper contacts thedensified layer (if included) or acoustical layer and the foil serves asthe outer surface of the backing.

In the event that paper is selected for the backing sheet, it may beconvenient to use conventional wallboard paper so that a single type ofpaper may be produced and installed on a line. The same type of papercan accommodate production of both wallboard and acoustical panelsaccording to the invention. For example, as will be appreciated by oneof ordinary skill in the art, conventional wallboard paper can be in theform of, for example, 6-8 ply paper as prepared in a cylinder machine,or 1-4 ply paper as prepared in a Fourdrinier paper process. Lower plypaper (e.g., as made according to a Fourdrinier process), preferably atleast 2 ply paper, is preferred because it reduces weight and enhancescutability. Where such lower ply paper is used, as will be appreciated,it may be desirable to employ powered unwinders for the paper roll tominimize tension as the paper unwinds so that the lower ply paperprovides the requisite strength and support.

It is noteworthy that the panels according to the invention preferablyare unbalanced such that the face side of the panel does not containsuch a sheet, unlike conventional gypsum wallboard, which contains paperon both the face and back surfaces of the board. However, although lessdesirable, in some embodiments of the invention, acoustical panel can beprovided with a sheet, such as paper, on the face side, which can bediscarded prior to drying in the kiln, or alternatively, an acousticallytransparent sheet can remain on the face side of the finished panel,particularly if perforations, preferably pin-hole sized, are providedthrough the sheet. For example, the acoustically transparent sheet maybe provided as a decorative surface as will be appreciated by oneskilled in the art. Suitable facing sheet materials include, forexample, perforated vinyl, perforated or acoustically transparent paper,nonwoven scrims, woven fabric or cloth materials, and the like. In someembodiments, the backing sheet and/or facing sheet, if present, can bepre-coated with a binder such as a pregelatinized starch to enhance thebond between the backing and/or facing sheets on the one hand and theinterlocking matrix of set gypsum of the acoustical layer and/ordensified layer on the other hand.

Preferably, a densified layer precursor, comprising calcined gypsum andwater, is applied onto the backing sheet. The densified layer is formedafter the densified layer precursor is cured such that an interlockingmatrix of set gypsum is formed. The densified layer can be similar incomposition to the acoustical layer, except that the densified layer ismuch more dense, usually because less foam voids are present. The gypsumslurry stream for forming the densified layer can be taken from the samemixer used to form the acoustical layer precursor. Preferably, however,the mixture for forming the densified layer precursor will not includethe same amount of foam as the mixture used to form the acoustical layerprecursor. The gypsum slurry stream for forming the densified layer isapplied onto the backing sheet upstream of the discharge of theacoustical layer precursor from the mixer. For purposes of the presentapplication, when weight percent ranges for ingredients are provided, itwill be understood that the specified amounts are by weight of thesolids content in the gypsum slurry mixture for forming the acousticallayer precursor, and may also apply to the gypsum slurry for forming thedensified layer precursor if both streams are taken from the same mixerand such additives are added in the mixer. A screed bar or the like canbe used to achieve a desired thickness for the densified layer.

To achieve its relatively high density, desirably, the densified layeris prepared so as to form a minimum of foam voids. Thus, in preferredembodiments, foam is added to the gypsum slurry after it exits the mixerthrough a discharge outlet such as, for example, a tangential dischargeconduit or bottom discharge conduit as are known in the art. See, forexample, commonly assigned U.S. Pat. Nos. 5,683,635 and 6,494,609. Aswill be appreciated by those skilled in the art, in preferredembodiments where the foam is added in the discharge conduit, a gypsumslurry stream for forming the densified layer precursor can be tappedfrom the mixer from one or more outlets such that little or no foamingagent is included in the densified layer precursor. In otherembodiments, where the foaming agent is added in the mixer, it isdesirable to employ one or more secondary mixers (often referred to inthe art as “edge mixers” since they generally are used in the formationof hard edges for wallboard) to beat foam out of the mixture used forforming the densified layer precursor so that the densified layer can beformed on the backing sheet. The precise location for the outlet(s) inthe mixer for the slurry for forming the densified layer will varydepending upon the particular configuration of each manufacturing line,and can be decided upon by those of ordinary skill in the art.

Furthermore, as will be appreciated by one of ordinary skill in the art,foaming agent generally is pre-generated such that it is added to thegypsum slurry while carried in water. Thus, it will be appreciated thataddition of the foaming agent into the mixer discharge is preferable forthe additional reason that the densified layer will be formed with alower water to calcined gypsum ratio than the gypsum slurry that formsthe acoustical layer such that the density of the densified layer isincreased because the evaporated water void volume will be less thanthat of the acoustical layer.

Aside from the foaming agent, to the extent that other components areincluded in the gypsum slurry used to form the acoustical layer, suchas, for example, cellulosic fiber, lightweight aggregate, binder, foamstabilizer, accelerator, water reducing agent, and/or an enhancingmaterial for improving resistance to permanent deformation, theirpresence in the gypsum slurry used to form the densified layer isacceptable. While such ingredients are not needed in the densifiedlayer, if it is desired to include them in the gypsum slurry for formingthe acoustical layer, it may be convenient to include such ingredientsin the mixer such that they ultimately will be included in both thegypsum slurry stream for forming the acoustical layer precursor and thegypsum slurry stream for forming the densified layer precursor. However,in some embodiments, some or all of the ingredients may be inserted intothe discharge from the mixer, similarly to the foam, so that theirpresence is eliminated or minimized in the densified layer.

The densified layer desirably enhances wet strength (e.g., flexuralstrength) to enhance handleability on the moving surface of themanufacturing line, as well as dry strength, and cutability so thatacoustical panel of the invention can be installed readily withoutbreaking under normal wear and tear during installation. Thus, ifpresent, the densified layer desirably improves the strength of thefinished panel. The densified layer can be relatively thin as comparedto the acoustical layer since the densified layer is not necessary toimpart acoustical value to the panel. By way of example, the densifiedlayer can have a thickness of from about 0.05 inches to about 0.3inches, more preferably, from about 0.125 inches to about 0.25 inches,still more preferably from about 0.175 inches to about 0.225 inches,even more preferably, about 0.2 inches. The densified layer preferablyhas a density of at least about 30 lbs/ft³, more preferably a densityfrom about 35 lbs/ft³ to about 50 lbs/ft³, even more preferably, fromabout 38 lbs/ft³ to about 46 lbs/ft³, and still more preferably, fromabout 40 lbs/ft³ to about 45 lbs/ft³.

A scrim layer optionally can be included in acoustical panel. Inembodiments where a densified layer is included in acoustical panel, thescrim layer preferably is disposed between the densified layer and theacoustical layer. In preparation of the panel, the scrim layerpreferably is applied onto the densified layer precursor. The scrimlayer preferably is included to further enhance strength and cutability,and to further support the acoustical layer precursor and the acousticallayer. Desirably, the scrim layer is selected so as to have expansionproperties compatible with the backing sheet to prevent warping undervarying humidity conditions, as will be appreciated by one of ordinaryskill in the art. When acoustical panel includes the densified layer andthe scrim layer, the panel will comprise, generally, the followingstructure: acoustical layer, scrim layer, densified layer, backingsheet.

The scrim layer, if included, preferably is porous to facilitateattachment of the scrim layer to the acoustical layer and the densifiedlayer, respectively, and to enhance drying of the densified layerprecursor. Where the scrim layer is not porous, drying time for thedensified layer can be prolonged. Attachment of the scrim layer, in asandwich structure, to the densified layer and acoustical layer,respectively, may also be enhanced by use of a binder, such as byapplying a pregelatinized starch onto one or both surfaces of the scrimlayer. Binder can be included in the mixture used to form the acousticallayer precursor and the densified layer precursor. Binder can also besprayed onto the scrim layer and backing sheet.

By way of example, and not by way of limitation, the scrim layer can bein the form of paper commonly used in forming backing sheets, non-wovenfiberglass scrims, woven fiberglass mats, other synthetic fiber matssuch as polyester, and the like, and combinations thereof. Preferredtypes of paper include paper used on the back side of wallboard as wellas paper conventionally used on plaster-based board, as will beappreciated by one of ordinary skill in the art. Paper such as IMPERIAL®gypsum base face paper used with IMPERIAL® plaster, commerciallyavailable from USG, can be used. Desirably, the exterior plys of paperused for the scrim layer are not treated with a waterproofing agent.

The scrim layer, if included, is provided to enhance the tensile and/orflexural strength of the panel. For example, in some embodiments, thescrim layer is provided in a thickness of from about 0.003 inches toabout 0.02 inches (e.g., 0.013 inch).

The scrim layer, if included, is applied to the densified layerprecursor continuously and forms part of the continuous ribbon thatlater is cut into wet panel precursor of pre-selected dimensions.Configuring the manufacturing line to accommodate providing of the scrimlayer will vary depending upon the manufacturing line, and is wellwithin the skill of those skilled in the art. For example, the scrimmaterial can be in the form of a continuous roll that is unwound, withthe use of powered unwinders preferred. In some embodiments, after thedensified layer precursor is applied to the backing sheet, the scrimlayer is applied (e.g., rolled) onto the densified layer precursor, withthe gypsum slurry for forming the acoustical layer precursor thenapplied onto the scrim layer.

The continuous ribbon and wet panel precursors, respectively, exhibitsufficient green strength to withstand their own weight as well as therigors of the manufacturing line, such as vibrations. As used herein,“green strength” refers to strength during or after the setting process,before the ribbon or wet panel precursors are dried. To furtherfacilitate handleability of the ribbon on the manufacturing line, theweight itself per unit volume (density) of the continuous ribbon isminimized. Although the density of the continuous ribbon is not narrowlycritical, preferably, the acoustical layer of the continuous ribbon hasa maximum density during the process prior to drying of about 53 lb/ft³,more preferably, a maximum density of about 43 lb/ft³. The water tocalcined gypsum ratio is adjusted to reduce water content in order tominimize wet weight and drying time. In addition, it is believed that alower water-stucco ratio enhances formation of open cell voids in theset gypsum-containing acoustical layer when foaming agent, is used. Anopen cell structure enhances acoustical properties as compared withclosed cell voids. Preferably, the weight ratio of water to calcinedgypsum in the aqueous slurry preferably ranges from about 0.5:1 to about1.5:1. Preferably, the calcined gypsum is primarily a beta hemihydratein which case the water to calcined gypsum ratio is preferably fromabout 0.7:1 to about 1.5:1, more preferably, from about 0.7:1 to about1.4:1, even more preferably, from about 0.75:1 to about 1.2:1, and stillmore preferably from about 0.77:1 to about 1.1:1.

To minimize the water to calcined gypsum ratio, water reducing agentpreferably is added to the aqueous gypsum slurry (e.g., via a pump) toenhance the fluidity of the slurry. Any suitable water reducing agentcan be used. For example, the water reducing agent can be in the form ofany suitable plasticizer commonly used in gypsum wallboard manufacture.Polysulfonates, such as, for example, naphthalene sulfonates or thelike, carboxylate compounds (e.g., polycarboxylates) such as acrylatesor the like, and melamine compounds are preferred, with carboxylate andmelamine compounds even more preferred because they are non-coloredmaterials. For example, Diloflo GW, commercially available from GEOSpecialty Chemicals, Inc. is a suitable naphthalene sulfonate, andEthaCryl™ 6-3070, commercially available from Lyondell Chemical Company,is a suitable acrylate.

If present, the water reducing agent is included in the gypsum slurry inany amount sufficient to impart the desired fluidity. For example, thewater reducing agent may be included in the aqueous calcined gypsummixture in an amount up to about 1.5%, more preferably up to about 1.0%by weight of the solids content in the mixture, more preferably, in anamount of from about 0.2% to about 0.5% by weight of the solids contentin the mixture. It is to be noted that, in the event that any ingredientis added to the gypsum mixture as part of a solution, the weight amountsprovided herein refer to the amount of that particular ingredient, andnot the weight amount of the solution in which that particularingredient is included.

The calcined gypsum can be in the form of alpha calcium sulfatehemihydrate, beta calcium sulfate hemihydrate, water-soluble calciumsulfate anhydrite, or mixtures thereof. In preferred embodiments, thecalcined gypsum is in the form of beta calcium sulfate hemihydrate. Thecalcined gypsum is present in the gypsum slurry in any amount sufficientto allow for the formation of an interlocking matrix of set gypsum. Forexample, the calcined gypsum can be included in the aqueous calcinedgypsum mixture in an amount of from about 50% to about 95% by weight ofthe solid content in the mixture, preferably, in an amount of from about60% to about 94% by weight of the solids content in the mixture, morepreferably, in an amount of from about 88% to about 94% by weight of thesolids content in the mixture, still more preferably, in an amount offrom about 90% to about 93.6% by weight of the solids content in themixture.

In preferred embodiments, cellulosic fiber is added as a fibrousreinforcement into the aqueous gypsum slurry. In particular, thecellulosic fiber provides flexural strength in the dry finishedacoustical panel, while also enhancing ductility, stiffness, and greenstrength in the wet ribbon and wet panel precursor. It is believed thatthe cellulosic fiber bonds with the set gypsum to enhance theconnectivity or networking of the interlocking matrix of set gypsum inorder to make the continuous ribbon and/or panel less brittle and thusless susceptible to fracture on the manufacturing line.

The cellulosic fiber can be provided in any amount sufficient to impartthe desired fibrous reinforcement. In some embodiments, the cellulosicfiber is present in an amount of from about 1% to about 12% by weight ofthe solids content in the mixture, more preferably, in an amount of fromabout 3% to about 6% by weight of the solids content in the mixture.

Any suitable cellulosic fiber can be utilized. Preferably, however, theaverage length of the cellulosic fiber is less than about 3 mm, morepreferably, less than about 2 mm. Such cellulosic fiber lengths arepreferred to enhance the cutability of the continuous ribbon and/or thepanels. In this respect, longer cellulosic fiber lengths may becomefrayed upon cutting. For example, recycled newsprint or the like canhave desirable fiber lengths for use in the practice of the invention,as compared with old corrugated cardboard (OCC) or kraft paper (althoughOCC and/or kraft paper can be used in some embodiments, if desired,especially if the fiber lengths thereof are modified to fall within thepreferred ranges). As will be appreciated by one of ordinary skill inthe art, some commercially available paper fibers, such as, for example,Kayocel 1650 paper fiber available from American Fillers and Abrasives,are provided in a mixture with calcium carbonate in a 1:1 ratio. Thepresence of the calcium carbonate in such commercially available paperfiber products is advantageous because it prevents clumping of the paperfiber during processing.

Acoustical panel preferably is formulated so that it has desirablecutability characteristics. In this respect, preferably, dry acousticalpanel can be cut so that the ends of the cut panels have a smooth, even,aesthetically pleasing edge. Preferably, acoustical panel according tothe invention also can be readily scored and snapped. In this respect,by scoring the backing sheet (and facing sheet, if present), e.g., witha knife, the entire panel will snap smoothly, thereby allowing a user tofurther cut the panel with relative ease at a job site to configure thepanel to a desired size and shape. It is noteworthy that the ability ofacoustical panels according to the invention to readily score and snapis a significant advantage over conventional acoustical panel, whichrequire the panels to be cut all the way through. Most preferredacoustical panel according to the invention also accommodate a smooth,even cut (e.g., with a utility knife to form a “shadow-line” edge foraesthetic purposes) at a job site just prior to installation.

In preferred embodiments, lightweight aggregate is added to the aqueousgypsum slurry that forms the acoustical layer precursor. Lightweightaggregate desirably has a bulk density of about 10 lb/ft³ or less.Lightweight aggregate provides filler space to reduce the density andweight of acoustical panel of the invention. Examples of suitablelightweight aggregate include, but are not limited to, expandedpolystyrene (e.g., chopped expanded polystyrene), expanded vermiculite,expanded perlite, ceramic microspheres, resin microspheres, and thelike, or combinations thereof. Notably, lightweight aggregate with arelatively higher water demand (e.g., hydrophilic materials) are lessdesirable because more water is required to be present in the aqueousgypsum slurry to accommodate them. The amount of water beyond what isneeded for the stoichiometry of the hydration reaction of the calcinedgypsum is preferably minimized, in accordance with preferred embodimentsof the present invention, because the presence of water undesirablyincreases the wet weight of the continuous ribbon and/or wet panelprecursor. As a result, preferred lightweight aggregate according to theinvention should absorb as little water as possible. Accordingly,hydrophobic lightweight aggregate, such as, for example, expandedpolystyrene (e.g., chopped expanded polystyrene) or the like is mostpreferred.

As will be appreciated by one of ordinary skill in the art, conventionalpolystyrene resin beads (e.g., having a bulk density of about 40 lb/ft³)can be expanded with steam to form expanded polystyrene spheres. In thisrespect, the polystyrene resin beads can be heated and extruded into anyof a variety of shapes, and further processed, for example, by cycles ofsteam expansions followed by cooling/curing. Typically, 3 to 4 series ofexpansions through a steam expander are used to produce a lightweightmaterial with a cellular internal structure and a preferred density offrom about 0.2 lb/ft³ to about 0.4 lb/ft³. The expanded spheres can bein any of a variety of shapes, such as, for example, spherical shape,half sphere, E shape, check-mark shape, S shape, sometimes referred toas a “peanut-shape,” as commonly used in packaging applications, and thelike. The expanded polystyrene can be passed through a chopper orgrinder, and the outflow constrained by variously configured screens, toproduce the preferred chopped expanded polystyrene particle sizedistribution. Although the chopped expanded polystyrene may have anysuitable bulk density in accordance with the present invention,preferably it has a bulk density of from about 0.1 lb/ft³ to about 10lb/ft³, more preferably, from about 0.2 lb/ft³ to about 0.3 lb/ft³. Insome embodiments, at least about 90% of the chopped expanded polystyreneparticles will pass through a 5 mm mesh.

Lightweight aggregate can be provided in any amount sufficient toprovide the desired amount of filler space, reduction in density, andweight of acoustical panel. For example, lightweight aggregate can bepresent in an amount of from about 0.2% to about 35% by weight of thesolids content in the mixture. As will be appreciated by one of ordinaryskill in the art, when higher amounts of lightweight aggregate areincluded in the gypsum mixture, as with lightweight aggregate havingrelatively higher density, such as perlite, the amount of calcinedgypsum can be relatively lower within the preferred calcined gypsumamounts set forth herein. In preferred embodiments comprising choppedexpanded polystyrene having a bulk density of from about 0.2 lb/ft³ toabout 0.3 lb/ft³, the lightweight aggregate is provided in an amount offrom about 0.2% by weight of the solids content in the mixture to about3% by weight of the solids content in the mixture, more preferably, inan amount of from about 0.7% to about 3% by weight of the solids contentin the mixture.

Binder is also included in preferred embodiments of the aqueous gypsumslurry to enhance the strength and integrity of the interlocking setgypsum matrix in dry acoustical panel, and to promote adhesion of theinterlocking matrix of set gypsum and the backing sheet. Any suitablebinder or combination of binders may be used. Preferably, binders areselected so that some binder functions at the interface between paperand the interlocking matrix of set gypsum, and some binder functionswithin the interlocking matrix of set gypsum. By way of example, thebinder may be in the form of a starch such as, for example, anunmodified corn or wheat starch, latex such as, for example, polyvinylacetate, acrylic, and styrene butadiene latexes, or combinationsthereof. A preferred binder according to the present invention is anacrylic binder, such as, for example, a self cross-linking acrylicemulsion. An example of a self cross-linking acrylic emulsion isRHOPLEX® HA-16, commercially available from Rohm and Haas. Where acrylicbinder is included, it preferably is included in an amount of from about0.5% to about 3% by weight of the solids content in the mixture, morepreferably about 1% by weight of the solids content in the mixture.

A starch binder can be included in the gypsum slurry used to form theacoustical layer precursor or the densified layer precursor. Migratingand non-migrating starches are known to those skilled in the art ofgypsum wallboard manufacture. Migrating starches can be included in theslurry used to form the acoustical layer precursor and densified layerprecursor used to make acoustical panel of the invention where theacoustical panel includes paper on both faces of the acoustical layerprecursor or on both faces of the densified layer precursor. As eitheror both precursor layer is cured, the migrating starch will have atendency to migrate toward the interface of paper and gypsum, and as thegypsum sets, the binder will improve adherence of the paper to the setgypsum. Migrating starches do not necessarily improve attachment of thepaper to set gypsum when paper is on only one surface of the gypsumbecause the migrating starch tends to migrate away from the paper inthat instance. Non-migrating starches can be included in the slurry toenhance the strength of the acoustical layer and densified layer ifpresent, and also can be applied in a solution directly onto paper toenhance bonding to gypsum.

For example, if present, starch can be provided in an amount of fromabout 0.8% to about 1.5% by weight of the solids content in the mixture.Examples of readily available pregelatinized non-migrating starches thatserve the purposes of the present invention are (identified by theircommercial names): GemGel starch, commercially available from ManildraGroup USA; and PCF1000 starch, available from Lauhoff Grain Co. Examplesof readily available non-pregelatinized, non-acid thinned, non-migratingstarches that serve the purposes of the present invention are(identified by their commercial names): Minstar 2000, available fromMinnesota Corn Products Company; and Clinton 106 Corn Starch, availablefrom ADM Company. Examples of readily available migrating starches thatserve the purposes of the present invention are (identified by theircommercial names): Hi-Bond starch, commercially available from LauhoffGrain Co. and LC-211 starch, commercially available from ADM Company.

When binder is used in a preferred practice of the invention, the binderpreferably is included in the aqueous calcined gypsum slurry in acumulative amount of from about 0.5% to about 5% by weight of the solidscontent in the mixture, more preferably, in an amount of from about 0.5%to about 2% by weight of the solids content in the mixture, still morepreferably, in an amount of from about 0.5% to about 1.5%.

In accordance with the present invention, one or more foaming agents arepreferably provided in order to impart voids in the setgypsum-containing product to enhance the acoustical properties andprovide lighter weight. Any of the conventional foaming agents known tobe useful in preparing foamed set gypsum products can be employed. Manysuch foaming agents are well known and readily available commercially,e.g., from GEO Specialty Chemicals in Ambler, Pa. For furtherdescriptions of useful foaming agents, see, for example, U.S. Pat. Nos.4,676,835; 5,158,612; 5,240,639 and 5,643,510 and PCT InternationalApplication Publication WO 95/16515, published Jun. 22, 1995.

Preferably, the foam is selected so that it forms a stable foam cell inthe acoustical layer of the acoustical panel. In this respect, it isbelieved that acoustical properties are enhanced as the surface area offoam voids in the final product increases and where the voidsinterconnect so as to form an open cell. By way of illustration, thefoam voids can have an average diameter of less than about 200 μm, lessthan about 100 μm, or even less than about 75 μm. The population of foamvoids in the acoustical layer is preferably such that a preponderance offoam voids has a diameter at or near the average diameter. The averagediameter of foam voids and the population of foam voids can be evaluatedby scanning electron microscopy (SEM) at a magnification of about 100×.Preferably, the acoustical layer has a foam void volume of from about35% to about 60%, more preferably from about 40% to about 55%, stillmore preferably from about 45% to about 50%.

It is preferred to use a stable foaming agent such as an alkyl ethersulfate, sodium laureth sulfate such as STEOL®CS-230, commerciallyavailable from Stepan. STEOL®CS-230 is a sodium laureth sulfate derivedfrom fatty alcohols, ethoxylated to an average of two moles, andsulfated via a continuous SO₃ process. Where an alkyl ether sulfate ischosen, preferably it is characterized by an average of at least 2-4ether units between alkyl and sulfate units.

An example of one type of foaming agent, useful to generate stablefoams, has the formula[CH₃(CH₂)_(x)CH₂(OCH₂CH₂)_(Y)OSO₃]Θ M+  (Formula II)wherein X is a number from 2 to 20, Y is a number from 0 to 10 and isgreater than 0 in at least 50 weight percent of the foaming agent, and Mis a cation.

The foaming agent is provided in an amount sufficient to achieve thedesired acoustical characteristics in the panel. For example, thefoaming agent can be present in an amount of from about 0.003% to about0.4% by weight of the solids content in the mixture, more preferably, inan amount of from about 0.005% to about 0.03% by weight of the solidscontent in the mixture, still more preferably from about 0.009% to about0.015% (e.g., 0.014%) by weight of the solids content in the mixture.

Although not required, if desired, in some embodiments, foam stabilizermay be added to the aqueous calcined gypsum slurry. An example of asuitable foam stabilizer such as an amido amine type of amine oxide(such as lauramidoproplyamine/Myristamidopropylamine Oxide, commerciallyavailable from Stepan as AMMONYX®LMDO) and/or cocamide DEA, such asNINOL®40-CO, commercially available from Stepan. If included, the foamstabilizer can be present in any suitable amount normally found when itis included during the manufacture of gypsum wallboard. In oneembodiment, foam and foam stabilizer is included in a weight ratio of80:10:10 of STEOL®CS-230: AMMONYX®LMDO:NINOL®40-CO.

Preferably, an accelerator for accelerating the hydration of calcinedgypsum to calcium sulfate dihydrate is included in the aqueous gypsumslurry. Any suitable accelerator may be used in the practice of theinvention, as are generally known in the manufacture of gypsumwallboard. By way of example, finely ground (e.g., less than about 25μm) calcium sulfate dihydrate (i.e., “gypsum seeds”), which are known toenhance nucleation of set gypsum crystals so as to increase thecrystallization rate thereof, can be used. As will be appreciated by oneof ordinary skill in the art, to promote heat resistance, the gypsumseeds may be coated with a known coating agent such as a sugar (e.g.,sucrose, dextrose, or the like), starch, boric acid, long chained fattycarboxylic acid, including salts thereof, or combinations thereof. Otherknown accelerators include, but are not limited to, sulfate salts suchas aluminum sulfate, potassium sulfate, sodium hydrogen sulfate, andcombinations thereof, and acids such as sulfuric acid.

The accelerator can be provided in any suitable amount. For example, theaccelerator can be present in an amount of from about 1% to about 15% byweight of the solids content in the mixture, more preferably, in anamount of from about 2% to about 8% by weight of the solids content inthe mixture, still more preferably, in an amount of from about 3% toabout 5% by weight of the solids content in the mixture.

Preferably, one or more enhancing materials are included in the aqueouscalcined gypsum mixture, in order to promote strength and/or dimensionalstability (e.g., by minimizing shrinkage due to drying stresses) duringpreparation of the inventive acoustical panels. Desirably, the enhancingmaterials are selected so that they do not retard the rate of, orotherwise adversely affect, the formation of the set gypsum. By way ofexample, the enhancing material can be selected from a trimetaphosphatecompound, an ammonium polyphosphate having 500-3000 repeating phosphateunits, and a tetrametaphosphate compound, including salts or anionicportions of any of the foregoing. Notably, a hexametaphosphate compound(e.g., sodium hexametaphosphate) having 6-27 repeating phosphate unitsmay be used to enhance sag resistance, if desired, although they are notas a beneficial because they have been found to reduce strength and haveretardive effects on the rate of hydration of the calcined gypsum. Oneor more of each type of enhancing material can be used in the practiceof the invention, if desired. See, e.g., commonly assigned U.S.applications Ser. No. 09/249,814, filed on Feb. 16, 1999 and Ser. No.10/015,066 filed on Dec. 11, 2001.

The use of a trimetaphosphate compound (e.g., salt or anionic portionthereof) is particularly preferred. Inclusion of the trimetaphosphatecompound during the hydration of calcined gypsum to form set gypsumresults in enhanced strength, including resistance to mechanicaldeformation (e.g., sag), of the set gypsum. The trimetaphosphatecompound can be, for example, in the form of a salt, e.g., sodiumtrimetaphosphate, aluminum trimetaphosphate, potassium trimetaphosphate,ammonium trimetaphosphate, lithium trimetaphosphate, or the like.Combinations of these salts can also be used. In some embodiments, thetrimetaphosphate compound is sodium trimetaphosphate.

The enhancing material can be added to the aqueous slurry in anysuitable amount, such as, for example, an amount of from about 0.004% toabout 2% by weight of the solids content in the mixture, morepreferably, in an amount of from about 0.1% to about 0.3% by weight ofthe solids content in the mixture.

In some embodiments, a dry mix comprising calcined gypsum, cellulosicfiber, lightweight aggregate, accelerator, and binder is formed. The drymix is metered into the main mixing chamber, where it is mixed with thewater. The foaming agent preferably is added as pregenerated, althoughin some embodiments, the foam may be generated in situ, if desired. Aswill be readily apparent to one of ordinary skill in the art, foam maybe pregenerated by mixing foaming agent, air, and water in a high shearfoam mixing apparatus such that the pregenerated foam is then deliveredto the mixer. The foaming agent, water reducing agent, enhancingmaterial, and foam stabilizer preferably are added in fluid form to themixer or into the discharge of the mixer, for example, as described inU.S. Pat. No. 5,683,635. For example, they can be delivered (e.g., via apump) separately, together in various combinations, or by entry into thewater line that feeds the mixer.

Turning now to the properties of acoustical panel of the invention,acoustical panel have an inherently acoustically absorptive, porous,acoustical layer structure, that is, without the need for holes to beadded mechanically. The acoustical panel according to preferredembodiments of the invention results in desirable acoustical propertiessuch that a desirable Noise Reduction Coefficient can be achievedaccording to ASTM C 423-02, where sound absorption is measured in areverberation room by measuring decay rate. A desirable Normal IncidentSound Absorption also can be achieved in accordance with a modified ASTME1050-98, in which the Normal Incident Sound Absorption is measured inan impedance tube from the average of four frequencies, i.e., 250, 500,1000 and 1600 Hz. The ASTM E 1050-98 is “modified” because the fourthfrequency is 1600 Hz, not 2000 Hz. The sample is tested without abacking air space, i.e., with the acoustical panel resting on a flatmetal surface, on a Brüel & Klaer Pulse™ Material testing systemconsisting of Pulse™ Material testing Program Type 7758, Two-microphoneImpedance Measurement Tube Type 4206 (400 mm diameter), Power AmplifierType 2706 and Pulse™, the Multi-analyzer System Type 3560. Preferably,acoustical panels according to the invention exhibit a Normal IncidentSound Absorption of at least about 0.32, pursuant to the modified ASTM E1050-98. More preferably, acoustical panels according to the inventionexhibit a Normal Incident Sound Absorption at least about 0.35, stillmore preferably, at least about 0.39, even more preferably, at leastabout 0.42, still more preferably, at least about 0.45, and even morepreferably, at least about 0.49. The Normal Incident Sound Absorptiondescribed herein is for acoustical panel consisting of an acousticallayer itself, and it is for acoustical panel comprising an acousticallayer and other components, such as a backing sheet, densified layer,scrim layer, and combinations thereof.

Preferably, the acoustical panel exhibits a Noise Reduction Coefficientof at least about 0.5, according to ASTM C 423-02, and more preferably,a Noise Reduction Coefficient at or near 1.0. For example, in someembodiments, the inventive panel demonstrates a Noise ReductionCoefficient according to ASTM C 423-02 of at least about 0.55, even morepreferably a Noise Reduction Coefficient of at least about 0.6, stillmore preferably, a Noise Reduction Coefficient of at least about 0.7,even more preferably, a Noise Reduction Coefficient of at least about0.8, and still more preferably, a Noise Reduction Coefficient of atleast about 0.9. The Noise Reduction Coefficient described herein is foracoustical panel consisting of an acoustical layer itself, and it is foracoustical panel comprising an acoustical layer and other components,such as a backing sheet, densified layer, scrim layer, and combinationsthereof.

In preferred embodiments, acoustical panels according to the inventionexhibit a flexural strength pursuant to a modified ASTM C367-99 of atleast about 100 psi, more preferably, at least about 120 psi. In thisrespect, ASTM C 367-99 was modified such that the sample was 3 incheswide by 10 inches long and using an 8 inch span. Preferably, acousticalpanels according to the invention exhibit a surface hardness pursuant toASTM C 367-99 of at least about 100 lbs, more preferably, at least about200 lbs; and a surface burning rating of Class A, according to ASTM E84-01.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

While this invention has been described with an emphasis upon preferredembodiments, it will be apparent to those of ordinary skill in the artthat variations of the preferred embodiments may be used and that it isintended that the invention may be practiced otherwise than asspecifically described herein. Accordingly, this invention includes allmodifications encompassed within the scope of the invention as definedby the following claims.

1. A continuous method for preparing acoustical panel comprising:forming a mixture comprising water and calcined gypsum; adding foamingagent to the aqueous calcined gypsum mixture; casting the mixture in acontinuous ribbon; maintaining the ribbon under conditions sufficientfor the calcined gypsum to form an interlocking matrix of set gypsum;cutting the ribbon to form wet acoustical panel precursor; and dryingthe wet panel precursor to form acoustical panel, wherein acousticalpanel has a Normal Incident Sound Absorption of at least about 0.32,according to a modified ASTM E 1050-98.
 2. The method of claim 1,wherein the mixture comprises cellulosic fiber.
 3. The method of claim2, wherein the cellulosic fiber is paper fiber.
 4. The method of claim2, wherein the amount of cellulosic fiber is from about 1% to about 12%by weight of the solids content in the mixture.
 5. The method of claim2, wherein the cellulosic fiber has an average fiber length of less thanabout 2 mm.
 6. The method of claim 1, wherein the mixture compriseslightweight aggregate.
 7. The method of claim 6, wherein the lightweightaggregate is expanded polystyrene.
 8. The method of claim 6, wherein thelightweight aggregate has an average particle size of from about 0.5 mmto about 5 mm.
 9. The method of claim 6, wherein the lightweightaggregate has a bulk density of from about 0.2 lb/ft³ to about 0.3lb/ft³.
 10. The method of claim 6, wherein the amount of lightweightaggregate is from about 0.2% to about 35% by weight of the solidscontent in the mixture.
 11. The method of claim 1, wherein the mixturecomprises binder.
 12. The method of claim 11, wherein the binder isselected from the group consisting of starch, latex, and combinationsthereof.
 13. The method of claim 12, wherein the latex is selected fromthe group consisting of an acrylic compound, polyvinyl acetate, styrenebutadiene, and combinations thereof.
 14. The method of claim 12, whereinthe starch is migrating.
 15. The method of claim 12, wherein the starchis non-migrating.
 16. The method of claim 12, wherein the starchcomprises a combination of migrating starch and non-migrating starch.17. The method of claim 11, wherein the amount of binder is from about0.5% to about 5% by weight of the solids content in the mixture.
 18. Themethod of claim 1, wherein the mixture is substantially free of mineralwool.
 19. The method of claim 1, wherein a face sheet is applied on themixture.
 20. The method of claim 1, wherein the amount of calcinedgypsum is from about 50% to about 95% by weight of the solids content ofthe mixture.
 21. The method of claim 1, wherein the mixture compriseswater reducing agent.
 22. The method of claim 21, wherein the waterreducing agent is selected from the group consisting of naphthalenesulfonates, polycarboxylate compounds, melamine compounds, andcombinations thereof.
 23. The method of claim 21, wherein the amount ofwater reducing agent is from about 0.2% to about 1.5% by weight of thesolids content in the mixture.
 24. The method of claim 1, wherein thefoaming agent forms an open cell structure in the acoustical panel. 25.The method of claim 24, wherein the foaming agent is characterized bythe formula[CH₃(CH₂)_(x)CH₂(OCH₂CH₂)_(Y)OSO₃]ΘM+ wherein X is a number from 2 to20, Y is a number from 0 to 10 and is greater than 0 in at least 50weight percent of the foaming agent, and M is a cation, and combinationsthereof.
 26. The method of claim 24, wherein the amount of foaming agentused in forming the mixture is from about 0.005% to about 0.4% by weightof the solids content of the mixture.
 27. The method of claim 1, whereinthe mixture comprises an accelerator.
 28. The method of claim 27,wherein the accelerator comprises calcium sulfate dihydrate.
 29. Themethod of claim 27, wherein the amount of accelerator used in formingthe mixture is from about 1% to about 15% by weight of the solidscontent of the mixture.
 30. The method of claim 1, wherein the mixturecomprises an enhancing material selected from the group consisting of anammonium polyphosphate having 500-3000 repeating phosphate units, atrimetaphosphate compound, a tetrametaphosphate compound, ahexametaphosphate compound, and combinations thereof.
 31. The method ofclaim 30, wherein the enhancing material is sodium trimetaphosphate. 32.The method of claim 30, wherein the amount of enhancing material is fromabout 0.004% to about 2% by weight of the solids content of the mixture.33. The method of claim 1, wherein the ribbon before drying has amaximum density of about 53 lb/ft³.
 34. The method of claim 1, whereinthe weight ratio of water to calcined gypsum in the mixture is fromabout 0.5:1 to about 1.5:1.
 35. The method of claim 1, furthercomprising applying a forming plate or forming rollers to the mixture asit is cast in the continuous ribbon.
 36. The method of claim 35, whereinthe forming plate is a fluidization membrane.
 37. The method of claim35, wherein the forming plate is a vibrating plate.
 38. The method ofclaim 1, wherein the mixture is cast directly or indirectly onto abacking sheet.
 39. The method of claim 38, wherein the mixture forforming the acoustical layer is cast directly on the backing sheet. 40.The method of claim 38, wherein the backing sheet is formed from amaterial selected from the group consisting of non-woven glass face,metallic foil, paper, a laminate comprising paper and a metallic foil,and combinations thereof.
 41. The method of claim 38, further comprisingapplying a densified layer precursor, comprising calcined gypsum andwater, on the backing sheet.
 42. The method of claim 41, wherein thedensified layer, when cured, has a density of at least about 35 lbs/ft³.43. The method of claim 41, further comprising applying a scrim layer onthe densified layer.
 44. The method of claim 43, wherein the scrim layeris selected from the group consisting of paper, non-woven fiberglass,woven fiberglass, synthetic fiber, and combinations thereof.
 45. Acontinuous method for preparing acoustical panel comprising: providing abacking sheet; forming a first mixture comprising (a) water, (b)calcined gypsum, and (c) foaming agent, and optionally one or more ofthe following: (d) cellulosic fiber, (e) lightweight aggregate, (f)binder, (g) accelerator, (h) water reducing agent, and (i) enhancingmaterial selected from the group consisting of an ammonium polyphosphatehaving 500-3000 repeating phosphate units, a trimetaphosphate compound,a tetrametaphosphate compound, a hexametaphosphate compound, andcombinations thereof; forming a second mixture comprising (a) water, and(b) calcined gypsum, and optionally one or more of the followingingredients: (c) cellulosic fiber, (d) lightweight aggregate, (e)binder, (f) accelerator, (g) water reducing agent, and (h) an enhancingmaterial selected from the group consisting of an ammonium polyphosphatehaving 500-3000 repeating phosphate units, a trimetaphosphate compound,a tetrametaphosphate compound, a hexametaphosphate compound, andcombinations thereof; casting the second mixture onto the backing sheetto form a densified layer precursor; casting the first mixture as anacoustical layer precursor onto the densified layer precursor to form acontinuous ribbon; maintaining the ribbon under conditions sufficientfor the calcined gypsum in each of the densified layer precursor and theacoustical layer precursor to form an interlocking matrix of set gypsum;cutting the ribbon to form wet acoustical panel precursor; and dryingthe wet panel precursor to form the acoustical panel, wherein theacoustical panel has a Normal Incident Sound Absorption of at leastabout 0.32, according to a modified ASTM E 1050-98.
 46. The method ofclaim 45, further comprising applying a scrim layer onto the densifiedlayer precursor.
 47. The method of claim 46, wherein the scrim layer isselected from the group consisting of paper, non-woven fiberglass, wovenfiberglass, synthetic fiber, and combinations thereof.
 48. The method ofclaim 45, wherein the first mixture comprises: (a) from about 50% toabout 150% water; (b) from about 50% to about 95% calcined gypsum; (c)from about 1% to about 12% cellulosic fiber; (d) from about 0.2% toabout 35% lightweight aggregate; (e) from about 0.5% to about 5% binder;(f) from about 0.005% to about 0.4% foaming agent; (g) from about 1% toabout 15% accelerator; (h) from about 0.2% to about 1.5% water reducingagent; and (i) from about 0.004% to about 2% enhancing material, whereinthe foregoing amounts are by weight of the solids content in themixture.
 49. The method of claim 45, wherein the cellulosic fiber ispaper fiber and the lightweight aggregate is expanded polystyrene. 50.The method of claim 45, wherein the second mixture further comprisesfoaming agent, the method further comprising beating the second mixtureto minimize formation of foam voids.
 51. The method of claim 1, whereinthe acoustical panel has a density of from about 12 lb/ft³ to about 20lb/ft³.
 52. The method of claim 51, wherein the mixture comprisescellulosic fiber.
 53. The method of claim 52, wherein the cellulosicfiber is paper fiber.
 54. The method of claim 52, wherein the amount ofcellulosic fiber is from about 1% to about 12% by weight of the solidscontent in the mixture.
 55. The method of claim 52, wherein thecellulosic fiber has an average fiber length of less than about 2 mm.56. The method of claim 51, wherein the mixture comprises lightweightaggregate.
 57. The method of claim 56, wherein the lightweight aggregateis expanded polystyrene.
 58. The method of claim 56, wherein thelightweight aggregate has an average particle size of from about 0.5 mmto about 5 mm.
 59. The method of claim 56, wherein the lightweightaggregate has a bulk density of from about 0.2 lb/ft³ to about 0.3lb/ft³.
 60. The method of claim 56, wherein the amount of lightweightaggregate is from about 0.2% to about 35% by weight of the solidscontent in the mixture.
 61. The method of claim 51, wherein the weightratio of water to calcined gypsum in the mixture is from about 0.5:1 toabout 1.5:1.
 62. The method of claim 1, wherein the mixture comprisescellulosic fiber and lightweight aggregate.
 63. The method of claim 62,wherein the cellulosic fiber is paper fiber.
 64. The method of claim 62,wherein the cellulosic fiber has an average fiber length of less thanabout 2 mm.
 65. The method of claim 62, wherein the lightweightaggregate is expanded polystyrene.
 66. The method of claim 62, whereinthe lightweight aggregate has an average particle size of from about 0.5mm to about 5 mm.
 67. The method of claim 62, wherein the weight ratioof water to calcined gypsum in the mixture is from about 0.5:1 to about1.5:1.
 68. The method of claim 45, wherein the first mixture comprises(a) water, (b) calcined gypsum, (c) foaming agent, (d) cellulosic fiber,and (e) lightweight aggregate.
 69. The method of claim 68, wherein thecellulosic fiber is paper fiber.
 70. The method of claim 68, wherein theamount of cellulosic fiber is from about 1% to about 12% by weight ofthe solids content in the first mixture.
 71. The method of claim 68,wherein the cellulosic fiber has an average fiber length of less thanabout 2 mm.
 72. The method of claim 68, wherein the lightweightaggregate is expanded polystyrene.
 73. The method of claim 68, whereinthe lightweight aggregate has an average particle size of from about 0.5mm to about 5 mm.
 74. The method of claim 68, wherein the lightweightaggregate has a bulk density of from about 0.2 lb/ft³ to about 0.3lb/ft³.
 75. The method of claim 68, wherein the amount of lightweightaggregate is from about 0.2% to about 35% by weight of the solidscontent in the mixture.
 76. The method of claim 68, wherein the weightratio of water to calcined gypsum in the mixture is from about 0.5:1 toabout 1.5:1.
 77. The method of claim 45, wherein the acoustical panelhas a density of from about 12 lb/ft³ to about 20 lb/ft³.
 78. Acontinuous method for preparing acoustical panel comprising: (i) forminga mixture comprising (a) water, (b) calcined gypsum, and (c) cellulosicfiber; (ii) adding foaming agent in the form of a pregenerated foam tothe aqueous calcined gypsum mixture; (iii) casting the mixture in acontinuous ribbon; (iv) maintaining the ribbon under conditionssufficient for the calcined gypsum to form an interlocking matrix of setgypsum; (v) cutting the ribbon to form wet acoustical panel precursor;and (vi) drying the wet panel precursor to form acoustical panel;wherein acoustical panel has a density of from about 10 lb/ft³ to about25 lb/ft³ and an open cell structure sufficient to give rise to a NormalIncident Sound Absorption of at least about 0.32 according to a modifiedASTM E 1050-98.
 79. The method of claim 78, wherein the cellulosic fiberis paper fiber.
 80. The method of claim 78, wherein the amount ofcellulosic fiber is from about 1% to about 12% by weight of the solidscontent in the mixture.
 81. The method of claim 78, wherein thecellulosic fiber has an average fiber length of less than about 2 mm.82. The method of claim 78, wherein the mixture comprises expandedpolystyrene lightweight aggregate having an average particle size offrom about 0.5 mm to about 5 mm.
 83. The method of claim 78, wherein themixture comprises binder selected from the group consisting of starch,latex, and combinations thereof.
 84. The method of claim 78, wherein themixture is substantially free of mineral wool.
 85. The method of claim78, wherein the amount of calcined gypsum is from about 50% to about 95%by weight of the solids content of the mixture.
 86. The method of claim78, wherein the mixture comprises water reducing agent selected from thegroup consisting of naphthalene sulfonates, polycarboxylate compounds,melamine compounds, and combinations thereof.
 87. The method of claim78, wherein the amount of foaming agent used in forming the mixture isfrom about 0.005% to about 0.4% by weight of the solids content of themixture.
 88. The method of claim 78, wherein the mixture comprises acalcium sulfate dihydrate accelerator.
 89. The method of claim 78,wherein the mixture comprises an enhancing material selected from thegroup consisting of an ammonium polyphosphate having 500-3000 repeatingphosphate units, a trimetaphosphate compound, a tetrametaphosphatecompound, a hexametaphosphate compound, and combinations thereof. 90.The method of claim 78, wherein the weight ratio of water to calcinedgypsum in the mixture is from about 0.5:1 to about 1.5:1.
 91. The methodof claim 78, wherein the mixture for forming the acoustical layer iscast directly onto a backing sheet.
 92. The method of claim 91, furthercomprising applying a densified layer precursor, comprising calcinedgypsum and water, on the backing sheet.
 93. A continuous method forpreparing acoustical panel comprising: (i) forming a mixture comprising(a) water, (b) calcined gypsum, and (c) about 1% to about 12% by weightof the solids content in the mixture cellulosic fiber having an averagefiber length of less than about 2 mm; (ii) adding foaming agent to theaqueous calcined gypsum mixture; (iii) casting the mixture in acontinuous ribbon; (iv) maintaining the ribbon under conditionssufficient for the calcined gypsum to form an interlocking matrix of setgypsum; (v) cutting the ribbon to form wet acoustical panel precursor;and (vi) drying the wet panel precursor to form acoustical panel;wherein acoustical panel has a Normal Incident Sound Absorption of atleast about 0.32 according to a modified ASTM E 1050-98.
 94. The methodof claim 93, wherein the cellulosic fiber is paper fiber.
 95. The methodof claim 93, wherein the mixture comprises expanded polystyrenelightweight aggregate having an average particle size of from about 0.5mm to about 5 mm.
 96. The method of claim 93, wherein the mixturecomprises binder selected from the group consisting of starch, latex,and combinations thereof.
 97. The method of claim 93, wherein themixture is substantially free of mineral wool.
 98. The method of claim93, wherein the amount of calcined gypsum is from about 50% to about 95%by weight of the solids content of the mixture.
 99. The method of claim93, wherein the mixture comprises water reducing agent selected from thegroup consisting of naphthalene sulfonates, polycarboxylate compounds,melamine compounds, and combinations thereof.
 100. The method of claim93, wherein the amount of foaming agent used in forming the mixture isfrom about 0.005% to about 0.4% by weight of the solids content of themixture.
 101. The method of claim 93, wherein the mixture comprises acalcium sulfate dihydrate accelerator.
 102. The method of claim 93,wherein the mixture comprises an enhancing material selected from thegroup consisting of an ammonium polyphosphate having 500-3000 repeatingphosphate units, a trimetaphosphate compound, a tetrametaphosphatecompound, a hexametaphosphate compound, and combinations thereof. 103.The method of claim 93, wherein the weight ratio of water to calcinedgypsum in the mixture is from about 0.5:1 to about 1.5:1.
 104. Themethod of claim 93, wherein the mixture for forming the acoustical layeris cast directly onto a backing sheet.
 105. The method of claim 104,further comprising applying a densified layer precursor, comprisingcalcined gypsum and water, on the backing sheet.
 106. A continuousmethod for preparing acoustical panel comprising: (i) forming a mixturecomprising (a) water, (b) calcined gypsum, and (c) about 1% to about 12%by weight of the solids content in the mixture cellulosic fiber; (ii)adding foaming agent in the form of a pregenerated foam to the aqueouscalcined gypsum mixture; (iii) casting the mixture in a continuousribbon; (iv) maintaining the ribbon under conditions sufficient for thecalcined gypsum to form an interlocking matrix of set gypsum; (v)cutting the ribbon to form wet acoustical panel precursor; and (vi)drying the wet panel precursor to form acoustical panel; whereinacoustical panel has a density of from about 10 lb/ft³ to about 25lb/ft³ and an open cell structure sufficient to give rise to a NormalIncident Sound Absorption of at least about 0.32 according to a modifiedASTM E 1050-98.
 107. The method of claim 106, wherein the cellulosicfiber is paper fiber.
 108. The method of claim 106, wherein the mixturecomprises expanded polystyrene lightweight aggregate having an averageparticle size of from about 0.5 mm to about 5 mm.
 109. The method ofclaim 106, wherein the mixture comprises binder selected from the groupconsisting of starch, latex, and combinations thereof.
 110. The methodof claim 106, wherein the mixture is substantially free of mineral wool.111. The method of claim 106, wherein the amount of calcined gypsum isfrom about 50% to about 95% by weight of the solids content of themixture.
 112. The method of claim 106, wherein the mixture compriseswater reducing agent selected from the group consisting of naphthalenesulfonates, polycarboxylate compounds, melamine compounds, andcombinations thereof.
 113. The method of claim 106, wherein the amountof foaming agent used in forming the mixture is from about 0.005% toabout 0.4% by weight of the solids content of the mixture.
 114. Themethod of claim 106, wherein the mixture comprises a calcium sulfatedihydrate accelerator.
 115. The method of claim 106, wherein the mixturecomprises an enhancing material selected from the group consisting of anammonium polyphosphate having 500-3000 repeating phosphate units, atrimetaphosphate compound, a tetrametaphosphate compound, ahexametaphosphate compound, and combinations thereof.
 116. The method ofclaim 106, wherein the weight ratio of water to calcined gypsum in themixture is from about 0.5:1 to about 1.5:1.
 117. The method of claim106, wherein the mixture for forming the acoustical layer is castdirectly onto a backing sheet.
 118. The method of claim 117, furthercomprising applying a densified layer precursor, comprising calcinedgypsum and water, on the backing sheet.